A method for judging the presence or absence of leakage in the discharge prevention system of an evaporated fuel treatment apparatus is described in Japanese Patent Application Kokai No. Hei 7-12016. In this method, in cases where the detected tank pressure is a negative pressure that differs from atmospheric pressure by a predetermined value or greater, this indicates that purging is being performed in a normal evaporated fuel treatment system (in the case of ordinary engine operation). Accordingly, it is judged in such cases that the evaporated fuel treatment system is in a normal state, with no leakage of evaporated fuel from said system. In cases where the system is not judged to be normal in this judgment process, e.g., in cases where the internal pressure of the fuel tank remains stationary in the vicinity of atmospheric pressure for a predetermined period of time, it is considered that there is a possibility of leakage, and a negative pressure diagnostic process is actuated. In this process, the discharge prevention system is placed in a state of negative pressure, and the presence or absence of leakage is ultimately detected from the ability of the system to maintain this negative pressure.
Furthermore, an evaporated fuel treatment apparatus which is equipped with a bypass that bypasses the pressure adjustment valve in a charging passage that causes the fuel tank to communicate with the abovementioned canister, and which separately judges the presence or absence of leakage in the tank system on the fuel tank side of the bypass valve and the presence or absence of leakage in the canister system on the canister side of the bypass valve, is described in Japanese Patent Application Kokai No. Hei 9-317572. Judgment of the presence or absence of leakage in the tank system is accomplished as follows: Immediately after the engine is started, the bypass valve is opened so that the system is caused to move toward atmospheric pressure. If the shift in the pressure of the fuel tank in this case is greater than a predetermined value, it is judged that the tank system is in a normal state with no leakage. If there is leakage in the tank system, the pressure of the fuel tank prior to the starting of the engine will be more or less equal to atmospheric pressure, so that the shift in pressure is small.
As consideration for the environment has become more important, there has been a demand for stricter criteria for judging the presence or absence of leakage. However, the internal pressure in a fuel tank constantly varies due to various factors such as the temperature of the fuel, the extent to which excess fuel from the engine space is returned, the load conditions of the vehicle, and vibration, etc. As a result, difficulties have been experienced in the accurate detection of leakage caused by very small holes.
In cases where leakage is detected in spite of the fact that there is actually no leakage, the frequent lighting of a warning lamp, or other indicator lowers the practical utility of the vehicle. Conversely, if no leakage is detected in spite of the fact that leakage is actually occurring, evaporated fuel continues to be emitted into the atmosphere. Accordingly, an object of the present invention is to provide an evaporated fuel treatment apparatus that enables the correct detection of leakage caused by very small holes (e.g., holes with a diameter of 0.5 mm).